Home About us Contact
|
Home About us Contact | ||
|
|
||
N-Heterocyclic Carbene Ligands (n-heterocyclic + carbene_ligand)
Selected AbstractsImproved Chiral Olefin Metathesis Catalysts: Increasing the Thermal and Solution Stability via Modification of a C1 -Symmetrical N-Heterocyclic Carbene LigandADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 11-12 2009Jolaine Savoie Abstract Four new ruthenium-based olefin metathesis catalysts that possess an N-heterocyclic carbene (NHC) ligand with benzyl (Bn) or or n -propyl (n -Pr) N-alkyl groups have been prepared. The synthetic routes developed for the synthesis of the required dihydroimidazolium salts are general. Catalysts bearing larger NHC ligands with larger N-alkyl groups displayed improved thermal and solution state stability up to 80,°C. The reactivity of the new catalysts in ring-closing metathesis is directly related to the increased steric bulk of the NHC ligand. The new catalysts have been evaluated in desymmetrization reactions and the nature of the N-alkyl group of the NHC ligands has been shown to have an important effect on the observed enantioselectivities. [source] Palladium(II) and Gold(I) Complexes of a New O-Functionalized N-Heterocyclic Carbene Ligand: Synthesis, Structures, and Catalytic Application.CHEMINFORM, Issue 25 2007Lipika Ray Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF. [source] Function of an N-Heterocyclic Carbene Ligand Based on Concept of Chiral Mimetic.CHEMINFORM, Issue 17 2007Takafumi Arao Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF. [source] Mono-, Bi- and Tridentate N-Heterocyclic Carbene Ligands for the Preparation of Transition-Metal-Based Homogeneous CatalystsEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 13 2009Rosa Corberán Abstract This microreview focuses on the preparation of homogeneous catalysts of group 8,11 metals containing N-heterocyclic carbene ligands. The current status of the design of mono-, bi- and tridentate NHC ligands is reviewed through the description of the authors' own work. The catalytic applications of such complexes include hydrogen-borrowing, C,C coupling, reduction of double bonds and C,H activation reactions. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source] Dinuclear Titanium(IV) Complexes Bearing Phenoxide-Tethered N-Heterocyclic Carbene Ligands with cisoid Conformation through Control of HydrolysisEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 30 2007Dao Zhang Abstract In situ generated N-heterocyclic carbene salt derivative Na2(L) of 1,3-bis(4,6-di- tert -butyl-2-hydroxybenzyl)imidazolium bromide, [H3(L)]Br, reacted with 1 equiv. of TiBr4 at ,78 °C to give a titanium complex of the composition [(L)TiBr2(thf)] (1), while the reaction in a 2:1 ratio under the same conditions afforded bisligand titanium complex [(L)2Ti] (2). Two oxygen-bridged titanium dimers, {[(L)TiBr]2(,-O)} (4) and {[(L)Ti(,-O)]2} (5), were obtained by control of hydrolysis of 1 and [(L)Ti(CH2Ph)2] (3) in tetrahydrofuran and diethyl ether. The molecular structures of 2, 4, and 5 have been confirmed by X-ray single-crystal analysis. The phenoxide-functionalized NHC ligand adopts transoid conformation in mononuclear complex 2 but rare cisoid conformation in dinuclear complexes 4 and 5. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007) [source] Improved Turnover Numbers in Palladium-Catalyzed Bisdiene Cyclization-Trapping using N-Heterocyclic Carbene LigandsADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 15 2005James Abstract An optimized palladium-N-heterocyclic carbene catalyst system effects the palladium-catalyzed bisdiene cyclization-trapping with phenol at the 0.01% catalyst loading level with a TON of 7.6×103 and TOF of 280,h,1, values much higher than typically found for this and related carbocyclizations. The reaction scales well and the trans -substituted six-membered ring product is obtained in excellent yield on a 10-mmole scale without further optimization of the catalyst system or reaction conditions. [source] ChemInform Abstract: Matching the Chirality of Monodentate N-Heterocyclic Carbene Ligands: A Case Study on Well-Defined Palladium Complexes for the Asymmetric ,-Arylation of Amides.CHEMINFORM, Issue 19 2009Xinjun Luan Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source] Chiral Phosphino- and (Phosphinooxy)-Substituted N-Heterocyclic Carbene Ligands and Their Application in Iridium-Catalyzed Asymmetric Hydrogenation.CHEMINFORM, Issue 52 2006Steve Nanchen Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF. [source] Electronic Nature of N-Heterocyclic Carbene Ligands: Effect on the Suzuki Reaction.CHEMINFORM, Issue 40 2005Niloufar Hadei Abstract For Abstract see ChemInform Abstract in Full Text. [source] New Chiral N-Heterocyclic Carbene Ligands in Palladium-Catalyzed ,-Arylations of Amides: Conformational Locking through Allylic Strain as a Device for StereocontrolCHEMISTRY - A EUROPEAN JOURNAL, Issue 21 2010Yi-Xia Jia Dr. Abstract New Enders/Herrmann-type chiral N-heterocyclic carbene (NHC) ligands have been developed and applied in asymmetric palladium-catalyzed intramolecular ,-arylations of amides. The best ligands feature the bulky tert -butyl group and ortho -substituted aryl groups at the stereogenic centers. Aryl bromides readily react at room temperature and aryl chlorides at 50,°C. The highly enantiomerically enriched (up to 96,%,ee) 3-alkyl-3-aryloxindole products were obtained in generally high yields (>95,%) except in cases of steric congestion. The critical roles both of the bulky alkyl group and of the ortho -aryl substituent at the stereogenic center of the ligand were revealed in the crystal structure of a [Pd(,3 -allyl)(NHC-L*)(I)] complex. The ligand aryl location and orientation is fixed by conformational locking that minimizes A1,3 -strain and enables optimal transfer of chiral information. [source] X-Ray Photoelectron Spectroscopy and Reactivity Studies of a Series of Ruthenium CatalystsCHEMCATCHEM, Issue 1 2009Katarzyna Jarzembska Abstract X-Ray photoelectron spectroscopy (XPS) was applied to six selected ruthenium precatalysts. The XPS data obtained were compared against reactivity and structural results. The XPS data confirmed some dependencies such as the electron-donor properties of the substituents at the ruthenium center. Additionally, the data combined with structural and reactivity results explain the differences between the character of Grubbs and Hoveyda catalysts. It was found that changing the PCy3 ligand to OiPr (PCy3=tricyclohexylphosphane, OiPr=isopropoxy) has a major influence on relative electron-donating properties of the N-heterocyclic carbene ligand (NHC) and PCy3 groups, which was supported by the relative charges on the Ru center for the examined compounds. Moreover, the turnover frequency (TOF) of a selected example reaction decreased when introducing a NHC group in the case of Grubbs catalysts, but increased in the case of Hoveyda-type catalysts. The XPS data also explained the relative activity values of some catalysts (higher reactivity of nitro-Hoveyda than Hoveyda second-generation catalysts). However, the binding energies do not predict TOFs. Sole examination of the XPS data does not provide a base for reaching unambiguous and binding conclusions as to the relative reactivity of all the investigated systems. [source] Mono-, Bi- and Tridentate N-Heterocyclic Carbene Ligands for the Preparation of Transition-Metal-Based Homogeneous CatalystsEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 13 2009Rosa Corberán Abstract This microreview focuses on the preparation of homogeneous catalysts of group 8,11 metals containing N-heterocyclic carbene ligands. The current status of the design of mono-, bi- and tridentate NHC ligands is reviewed through the description of the authors' own work. The catalytic applications of such complexes include hydrogen-borrowing, C,C coupling, reduction of double bonds and C,H activation reactions. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source] Synthesis, Structural Diversity, and Ligand-Transfer Potential of (Carbene)copper(I) ComplexesHELVETICA CHIMICA ACTA, Issue 6 2009Galmari Venkatachalam Abstract Copper(I) complexes comprising different N-heterocyclic carbene ligands were prepared via in situ deprotonation and metallation. Depending on the wingtip groups on the carbene ligand (alkyl vs. aryl, chelating or monodentate), a variety of different structural motifs were identified, such as a trigonal planar geometry (alkyl wingtips) and an unprecedented see-saw-type structure (pyridinyl wingtip groups). While aryl wingtip groups increase the stability of the complexes, alkyl substituents induce rapid demetallation in the presence of moisture. The reactivity of these complexes was used to establish a carbene-transfer protocol, which is illustrated by the formation of new cyclic thiourea compounds (transfer to sulfur) and new (carbene)ruthenium(II) complexes (transfer to ruthenium). This suggests that (carbene)copper(I) complexes could become valuable alternatives to (carbene)silver(I) complexes for synthesizing (carbene)metal systems via transmetallation. [source] | ||||||||||||||||||||||